1. Field of the Invention
The subject invention is related to a cell-type filter cartridge, and more particularly, to a filtration cell having a very low tendency toward media buckling resulting from the inclusion therein of a thermoplastic edge seal having certain desirable mechanical characteristics.
2. Background of the Related Art
Cell-type filter assemblies, often referred to as lenticular filter assemblies, are well known in the art and have been employed in fluid processing systems for many years. An early example is disclosed in U.S. Pat. No. 2,788,901 to Boeddinghaus et al. Lenticular filter assemblies often include a plurality of vertically-stacked, co-axially arranged, filtration cells disposed within a cylindrical housing. The filtration cells are conventionally fabricated from two disc-shaped layers of filter media separated from each other such that liquid flows from the outside of the filter medium into the space between the layers of filtration media towards the central portion of the cell. The filter media discs are typically formed from cellulosic fibers. The separators are generally formed in the shape of discs with a plurality of ribs extending radially outward from a central aperture in a spoke-like pattern. An example of a lenticular filtration cell having such a construction is disclosed in U.S. Pat. No. 4,783,262 to Ostreicher et al., assigned to the assignee of the present application, the disclosure of which is herein incorporated by reference.
In prior art cells of this type, the two media discs are joined together by a thermoplastic edge seal which grips the two media layers to form a liquid tight seal at the outer periphery of the cell. U.S. Pat. No. 4,347,208 to Southall discloses a method of making a filtration cell having a sealed periphery which includes the steps of placing two media discs and an interposed separator into a mold and injecting a thermoplastic polymer into the mold to form a seal around the two media discs. The Southall patent discloses that polypropylene, polyethylene, nylon, and polysulfone are the preferred thermoplastic polymers for molding the edge seal, all of which have a relatively high elastic modulus.
It is known that thermoplastic edge seals formed by injection molding undergo radial shrinkage when cooled after solidification. This shrinkage induces a compressive radial stress in the filter media, thus increasing the tendency towards buckling in use, a condition wherein the cell media layers deflect in a vertical plane. Furthermore, conventionally such thermoplastic edges are molded around a dry, solid, media discs, resulting in a thermoplastic ring diameter larger than if thermoplastic material was molded and then allowed to cool unrestrained. Therefore the restraint from typical shrinkage during molding induces molded-in stress in the thermoplastic edge seal, causing a distortion of the cell during initial heat sterilization/sanitization cycles. The Southall patent recognizes that this is a characteristic of thermoplastic materials and discloses that the amount of polymer used during the injection molding process should be kept to a minimum to prevent excessive radial shrinkage of the edge seal and thereby decrease the tendency towards buckling.
It is well known in the art that the problem of cell buckling is exacerbated during hot wash cycles in which filtration cells are subjected to temperatures in excess of about 180xc2x0 F. During hot wash cycles, as the cell-type filter assemblies are heated and the filter media wet out, differential expansion coefficients between the thermoplastic components of the cell and the cell media result in increased compressive stresses in the media elements, further increasing the tendency towards buckling. During the period of time following a hot wash cycle, commonly referred to as the post flush period, the thermoplastic edge seal cools to room temperature and thus contracts radially inwardly while the cellulosic media elements, still wet, remain swollen in an expanded state. Consequently, there is an interference generated and an increased tendency towards buckling.
It would be beneficial, therefore, to provide a lenticular filtration cell with a molded thermoplastic edge seal which does not impose excessive compressive forces on the cell media once cooled after injection molding, during hot wash cycles, or during post flush periods.
The subject invention is directed to a lenticular filtration cell of the type having two filtration media elements, which optionally may be in the form of discs, a spacer element interposed, and joined at the outer peripheries thereof by a thermoplastic edge seal during an injection molding process. The media elements are formed from a cellulosic material and the spacer element is formed from a thermoplastic material which serves to maintain the structural integrity of the filtration cell.
In accordance with the subject invention, the thermoplastic material from which the edge seal is formed during the injection molding process has a relatively low modulus of elasticity, as compared to other thermoplastic materials, such as, for example, polypropylene, polysulfone and nylon. Therefore, the filtration cell is significantly less prone to media buckling than prior art cell-type filter assemblies as a consequence of stresses induced by the injection molding process, or those which are induced during a hot flush period in which the cell media and edge seal expand due to liquid absorption and elevated temperatures, or those which are induced during a post flush period in which the edge seal tends to contract as it cools while the cell media remains in an expanded state due to fluid absorption.
One aspect of the present invention includes a fluid filter cell comprising: at least two filtration media elements; a separator element operatively positioned between the at least two filtration media elements; and an edge seal operatively connecting the at least two filtration elements and the separator element together, the edge seal being formed from a material having a modulus of elasticity sufficient to enable the edge seal to expand when the filtration media elements expand while maintaining an effective seal there between.
Another aspect of the present invention includes in a fluid filtration cell including a separator disposed between two media elements having outer peripheries which are secured together by an edge seal, the media elements having a first set of dimensional characteristics in a dry state and a second set of dimensional characteristics in a wet state, the improvement comprising: an edge seal formed from a thermoplastic material which expands and contracts in response to changes in the dimensional characteristics of the media elements while maintaining an effective seal between the two media elements.
Yet another aspect of the present invention includes in a fluid filtration cell including a separator disposed between two media elements having outer peripheries which are secured together by an edge seal, the media elements being formed from a material which swells in size due to fluid absorption, the improvement comprising: an edge seal formed from a thermoplastic material having a modulus of elasticity which is sufficient to permit radial expansion of the edge seal when the cell media swells in size.
And yet another aspect of the present invention includes in a fluid filtration cell including a separator disposed between two media elements having outer peripheries which are secured together by an edge seal, the media elements being formed from a material which swells in size due to fluid absorption, the improvement comprising: an edge seal formed from a thermoplastic material having a modulus of elasticity which is sufficiently low to enable the edge seal to comply with the forces exerted thereupon by the media elements as the media elements swell in size.
Further features of the subject invention will become more readily apparent to those having ordinary skill in the art from the following description of the invention taken in conjunction with the drawings.